Switching power converters typically include one or more magnetic devices, such as inductors and/or transformers. Inductors store energy and are often used in switching power converters for filtering purposes. Similarly, transformers are typically designed to transfer energy in switching power converters, and achieve galvanic isolation and/or transform voltage magnitude.
It is known that a single coupled inductor can replace multiple discrete inductors in a switching power converter, to improve converter performance, to reduce converter size, and/or to reduce converter cost. Examples of coupled inductors and associated systems and methods are found in U.S. Pat. No. 6,362,986 to Schultz et al., which is incorporated herein by reference.
One known switching power converter is the current doubler. As known in the art, a current doubler includes a transformer and at least two inductors. The inductors can be implemented by a coupled inductor, and the coupled inductor can be combined with the transformer, so that all core magnetic devices of the current doubler are implemented in a single physical package.
FIG. 1 schematically illustrates a prior art current doubler 100 including an integrated transformer and coupled inductor 102, hereinafter referred to as “integrated magnetic device 102.” Integrated magnetic device 102 includes a first primary winding 104, a second primary winding 106, a first secondary winding 108, a second secondary winding 110, and a magnetic core 111. First primary winding 104 and second primary winding 106 are electrically coupled in series. One end of first primary winding 104 is electrically coupled to a first primary switching circuit 112 via a blocking capacitor 114, and one end of second primary winding 106 is electrically coupled to a second primary switching circuit 116. Blocking capacitor 114 helps prevent magnetic saturation of the transformer of integrated magnetic device 102 by charging to a voltage that causes direct current (DC) through first and second primary windings 104 and 106 to be zero. Each primary switching circuit 112 and 116 is electrically coupled between a positive input power node 118 and a negative input power node 120.
One end of each secondary winding 108 and 110 is electrically coupled to a positive output power node 122. The other end of each secondary winding 108 and 110 is electrically coupled to a negative output power node 124 via a respective switching device 126 and 128. First and second primary switching circuits 112 and 116 operate under the command of a controller 130 to transfer power from an input power source 132 to a load 134, as known in the art.
Magnetic saturation of integrated magnetic device 102 can also be prevented by using cycle-by-cycle primary-side current mode control, where controller 130 controls operation of current doubler 100 so that peak magnitude of current through primary windings 104 and 106 is the same in successive switching cycles applying alternating polarity voltage to the primary windings, thereby preventing ramping up of magnetizing current and resulting magnetic saturation. Blocking capacitor 114 can therefore be omitted in cases where controller 130 implements primary-side current mode control.
However, cycle-by-cycle primary-side current mode control may not prevent magnetic saturation of integrated magnetic device 102 when current doubler 100 is powering a high-frequency transient load. For example, consider a scenario where current doubler 100 is powering a transient load having a load step frequency similar to the switching frequency of current doubler 100, and assume that there is a decrease in load between successive first and second switching cycles. Controller 130 will increase switching duty cycle in the second switching cycle to compensate for the load decrease, so that peak current in integrated magnetic 102 in the second switching cycle is the same as that in the first switching cycle. This disparity between duty cycles of the first and second switching cycles, however, will cause magnetic flux imbalance in integrated magnetic device 102, potentially resulting in magnetic saturation.
Additionally, conventional secondary-side current mode control will not prevent a DC component from developing in current through first and second primary windings 104 and 106. Thus, conventional secondary-side current mode control will not prevent the transformer of integrated magnetic device 102 from saturating.
FIG. 2 illustrates integrated magnetic device 102. Integrated magnetic device 102 has a “ladder” configuration and includes rails 202 and 204, rungs 206 and 208, and a gapped leakage post 210. Rungs 206 and 208 and leakage post 210 are each disposed between rail 202 and rail 204. First primary winding 104 and first secondary winding 108 are wound around rung 206, and second primary winding 106 and second secondary winding 110 are wound around rung 208. Dashed line 212 illustrates the approximate coupling magnetic flux path within integrated magnetic device 102, where coupling magnetic flux is magnetic flux that links both secondary windings 108 and 110. Dashed line 214 illustrates the approximate leakage magnetic flux path of first secondary winding 108, where leakage magnetic flux of first secondary winding 108 is magnetic flux that links first secondary winding 108 but does not link second secondary winding 110. Dashed line 216 illustrates the approximate leakage magnetic flux path of second secondary winding 110, where leakage magnetic flux of second secondary winding 110 is magnetic flux which links second secondary winding 110 but not first secondary winding 108.
Integrated magnetic devices have been proposed in U.S. Pat. No. 7,417,875 to Chandrasekaran et al. (Chandrasekaran '875). The windings of these integrated magnetic devices extend outside of the magnetic core, and therefore, these integrated magnetic devices are prone to generate significant fringing magnetic flux, which may cause undesirable losses and electromagnetic interference. Additionally, Chandrasekaran 875's devices may be difficult to manufacture because the magnetic core blocks access to opposing sides of the windings during winding installation. Furthermore, Chandrasekaran 875's illustrated devices have relatively long coupling magnetic flux paths between some of the windings, which potentially results in large magnetic core losses. U.S. Pat. Nos. 6,873,237; 7,046,523; 7,280,026; 7,633,369; and 8,134,443 disclose magnetic devices similar to those of Chandrasekaran '875.